The present invention concerns optical scanners.
A scanner employs a mirror or other deflecting device that receives light from a stationary source and deflects it to cause a resultant moving light spot on an object surface. (It can also be used reciprocally, to collect light from a moving object spot and direct it to a stationary detector. In most of the description below, only the stationary-source organization will be discussed, but those skilled in the art will recognize that the invention is equally applicable to reciprocal arrangements, as well as to combinations of the two arrangements.)
In one type of optical scanning system, a source, such as a laser, shines light toward a first scanning mirror, which is pivoted about a first axis and reflects the light to a second scanning mirror, which is pivoted about a second axis substantially orthogonal to the first, and the light is thereby directed to an object spot in an object surface to be scanned. The angular positions of the two mirrors respectively determine the spot's x and y coordinates on the object surface.
The typical system further includes an objective lens in the resultant path, which focuses the light to a small object point on the object surface, thereby yielding high system resolution.
One of the problems posed by this arrangement is that its geometry results in different light-path lengths for different object points in an object plane. As a consequence, if the objective is a fixed-focal-length lens, a beam that is in focus at one object point will tend to be out of focus at other object points, and system resolution accordingly suffers.
Another difficulty posed by the geometry is the interaction between the two scanning mirrors. Because the distance from the second mirror to the object plane varies with that mirror's angular position, the amount of deflection of the target spot for a given angular position of the first mirror varies with the angular position of the second mirror, and this tends to result in so-called "pincushion" distortion of the resultant raster if no steps are taken to eliminate that effect.
Among the proposals for dealing with these problems is that contained in U.S. Pat. No. 4,750,045 to Ohara et al. Instead of employing a planar object surface, Ohara et al. employ an object surface substantially in the form of a cylinder section whose axis coincides with the pivot axis of the second mirror. This eliminates the path-length variation that results from the second mirror's pivoting, and it thereby eliminates pin-cushion distortion of the resultant raster. The cylindrical shape does not eliminate the path-length effects of the first mirror's pivoting, but Ohara et al. deal with this problem by placing between the two mirrors an objective of the field-flattening type, i.e., one whose focal length so varies with the angle at which light hits it that it compensates for the path-length change through a limited range of first-mirror pivot angles.
Unfortunately, this approach depends greatly on the imaging lens, which is ordinarily required to be a compound lens--typically, an f.multidot..theta. lens--of a relatively large number of elements if the field flattening is to be achieved satisfactorily through a reasonable range of angles.